C - Segmentation fault using Scatterv with dynamic 2D array

Question

I'm trying to work with 2D arrays and MPI_Scatterv. When I call MPI_Scatterv I get

    ================================================================================
    =   BAD TERMINATION OF ONE OF YOUR APPLICATION PROCESSES
    =   PID 5790 RUNNING AT ubuntu
    =   EXIT CODE: 139
    =   CLEANING UP REMAINING PROCESSES
    =   YOU CAN IGNORE THE BELOW CLEANUP MESSAGES
    ================================================================================
YOUR APPLICATION TERMINATED WITH THE EXIT STRING: Segmentation fault (signal 11)
This typically refers to a problem with your application.
Please see the FAQ page for debugging suggestions

If I use C99 2D arrays it works, but not with malloc. I want to know where I'm wrong with malloc. I can't use linearized 2D array, so I can't create array like array[i*columns+j]

Here is a test program:

int **alloc2d(int n, int m) {
      int i;
      int **array = malloc(n * sizeof(int*));
      array[0] = malloc(n * m * sizeof(int));
      for(i = 1; i < n; i++) 
          array[i] = array[i-1] + m;
      return array;
}

int *genSendc(int dim, int numprocs) {
    int* sendc = (int*)malloc(sizeof(int)*numprocs); 
    int i;
    int subsize = dim/numprocs;
    for(i=0; i<numprocs; ++i)
        sendc[i] = subsize;
    for(i=0; i<dim-subsize*numprocs; ++i)
        sendc[i]+=1;
    return sendc;
}

int *genDispl(int numprocs, int*sendc) {
    int* displ = (int*)malloc(sizeof(int)*numprocs);
    int i;
    displ[0]=0;
    for(i=1; i<numprocs; ++i)
        displ[i] = displ[i-1]+sendc[i-1];
    return displ;
}

int main(int argc, char *argv[]){
    int numprocs, rank, i, j, N=5, M=4;
    int* displMat, *sendcMat;
    int **txMatrix, **rxMatrix;

    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
    MPI_Comm_rank(MPI_COMM_WORLD,&rank);

    sendcMat = genSendc(N, numprocs);
    for(i=0; i<numprocs; ++i)
        sendcMat[i] *= M;
    displMat = genDispl(numprocs, sendcMat);

    rxMatrix = alloc2d(sendcMat[rank]/M, M);
    if (rank == 0) {
        srand(time(NULL));
        txMatrix = alloc2d(N, M);
        for (i=0; i < N; ++i)
            for(j=0; j < M; ++j)
               txMatrix [i][j] = (rand() % 10)+1;
    }

    MPI_Scatterv(&txMatrix[0][0], sendcMat, displMat, MPI_INT, &rxMatrix[0][0], sendcMat[rank], MPI_INT, 0, MPI_COMM_WORLD);

    MPI_Finalize();
}

If I print rxMatrix after MPI_Scatterv , the program prints Rank0 sub-matrix and then it crashes with segmentation fault. Where am I wrong?

Answer 1

This expression invokes undefined behavior if txMatrix is not properly initialized.

&txMatrix[0][0]

While the first argument to MPI_Scatterv is inconsequential on non-root ranks*, just evaluating the expression can cause a segfault. Just use an if/else for root/nonroot and pass NULL for the latter.

*: at least per the standard, I've seen this be bugged in MPI implementations.